Owing in particular to its excellent sensitivity and its good depth resolution, Secondary Ion Mass Spectrometry (SIMS) constitutes an extremely powerful technique for analysis of surfaces and thin films. Its main fields of application lie in the semiconductor, glass, organic and metallurgical composite materials.
SIMS instruments also allow the recording of ion images of the surface of the analysed sample. In this case, a thin ion probe sweeps the surfaces of the sample and the secondary ions which have suitably selected mass are recorded with respect to the position on the surface from which they originate. An emerging field of application for this imaging technique providing good lateral resolution combined with excellent sensitivity is situated in particular in biology.
Alongside all its advantages, however, the SIMS technique suffers from one major drawback: the measurements can only be quantified with difficulty. The intensity of the measured signals is generally greatly dependent on the sample analysed, given that the ionisation yield of a given sputtered element may vary by several orders of magnitude depending on the composition of the matrix in which it is located. This phenomenon is known as the matrix effect.
In order to get round these problems linked to the matrix effect, the SIMS analyses are increasingly carried out in MCsx+ mode. This method consists in incorporating cesium (Cs) in the material of interest and detecting positive ion clusters formed by the recombination of an atom of the element M in which one is interested together with one or two atoms of Cs. Given that these MCsx+ clusters are formed by atomic recombinations above the surface of the sample, the composition of the matrix does not come any longer into play directly, consequently eliminating the matrix effect.
SIMS instruments of prior art have exclusively used a beam made of Cs+ primary ions in order to perform analyses in the MCsx+ mode. A number of drawbacks are still to be listed with this respect.
Optimisation of the Cs Concentration to form MCsx+ Clusters
When the analyses in MCsx+ mode are carried out by bombarding the sample with a beam of Cs+ ions, this beam serves both for the incorporation of Cs in the material and for the sputtering of the surface. In this case, the Cs concentration, which is a crucial parameter determining the sensitivity of the analysis, as shown in FIG. 1, is set by the primary bombardment conditions—mainly the angle and energy of impact—which can be adapted only in a very limited way on conventional SIMS equipment. Consequently, the Cs concentration is practically fixed for a given type of sample and cannot be chosen freely. As it is unlikely that the Cs concentration thus obtained will coincide with the optimum concentration for the material in question, the analysis is not optimised.
Coupling of the Cs Concentration and the Depth Resolution
A second major disadvantage of the use of Cs+ ion bombardment relates to the impossibility of separately choosing the Cs concentration (cCs) implanted in the sample and the energetic and angular parameters of the primary beam, given that the latter determine the value of cCs. Now the primary bombardment conditions also considerably affect major analytical characteristics such as the depth resolution.
Pre-Equilibrium State
The introduction of Cs into the material by ion bombardment does not allow an optimum Cs concentration to be attained right from the first atomic layer, given that the Cs atoms are implanted under the surface, as shown in FIG. 2, at greater or lesser depth depending on their impact energy. Consequently, the analysis is inconclusive in the pre-equilibrium state which precedes the achieving of a constant concentration of Cs (in a Cs+ bombardment) or Cs and Ga (in a Cs+ and Ga+ bombardment).
Optimisation of Formation of Negative Secondary Ions
Furthermore, bombardments by electropositive elements are often used to raise the negative ion yield by several orders of magnitude. In this context, the emission of negative secondary ions is greatly enhanced by the presence of Cs atoms on the surface of the sample which is bombarded.
Aims of the Invention
The present invention aims at providing a new Secondary Ion Mass Spectroscopy (SIMS) method operating in the MCsx+ (x=1,2) mode permitting to separately choose the Cs concentration implanted in the sample and the sputtering of the sample surface, leading thus to simultaneous optimisation of the Cs concentration and analytical parameters, such as depth resolution, which depend now exclusively on primary bombardment conditions.
Particularly, the invention aims at permitting one, by depositing neutral Cs atoms on the sample surface, to vary the Cs concentration continuously in the range quasi 0 to 100% to an optimum value in order to maximise detected MCsx+ and Csx+ signals for any kind of sample.
Additionally, the invention aims at enabling an optimised signal to be measured right from the first atomic layer.
Another goal of the present invention is to provide a specially-developed cesium column with significant service life increase and designed to considerably reduce the risk of contaminating of the cesium deposit as well as of the analysis chamber with traces of other elements.